1. Field of the Invention
The present invention relates to a method for optimizing illumination in a lithographic apparatus. More specifically, the invention relates to a method for optimizing illumination using computer simulation.
2. Description of the Related Art
The term “patterning structure” as here employed should be broadly interpreted as referring to a device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device. An example of such a patterning structure is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase shift, and attenuated phase shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a patterning structure is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning structure can include one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT Publication Nos. WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a patterning structure is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., including one or more dies on a substrate (silicon wafer) that has been coated with a layer of radiation sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatuses, employing patterning by a mask on a mask table, a distinction can be made between two different types of machines. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step and scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally<1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792.
In a known manufacturing process using a lithographic projection apparatus, a
pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement and/or inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g., an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemical, mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. It is important to ensure that the overlay (juxtaposition) of the various stacked layers is as accurate as possible. For this purpose, a small reference mark is provided at one or more positions on the wafer, thus defining the origin of a coordinate system on the wafer. Using optical and electronic devices in combination with the substrate holder positioning device (referred to hereinafter as “alignment system”), this mark can then be relocated each time a new layer has to be juxtaposed on an existing layer, and can be used as an alignment reference. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing,” Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens.” However, this term should be broadly interpreted as encompassing various types of projection systems, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens.” Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatuses are described, for example, in U.S. Pat. Nos. 5,969,441 and 6,262,796.
Photolithography is widely recognized as one of the key steps in the manufacturing of integrated semiconductor circuits. At present, no alternative technology provides the desired pattern architecture with similar accuracy, speed, and economic productivity. However, as the dimensions of the ICs become smaller, photolithography is becoming one of the most, if not the most, critical gating factors for enabling miniature semiconductor device structures to be manufactured on a truly massive scale.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution R as shown in equation (1):
                    R        =                              k            1                    *                      λ            NA                                              (        1        )            
where λ is the wavelength of the radiation used, NA is the numerical aperture of the lens in the projection system and k1 is a process dependent adjustment factor.
It follows from equation (1) that the resolution can be improved in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1. All of these strategies have been pursued simultaneously in the past and are expected to continue in the future.
The performance of the lithographic apparatus and its limitation may also be explained and characterized with the Depth Of Focus (DOF), which is generally viewed as one of the most critical factors in determining the resolution of the lithographic projection apparatus. The DOF, defined in equation (2), is defined as the distance along the optical axis over which the image of the pattern is adequately sharp.
                    DOF        =                              +                          /                              -                                  k                  2                                                              *                      λ                          NA              2                                                          (        2        )            
where k2 is an empirical constant.
Additional important responses/measures that provide more insight into the real difficulties associated with photolithography at the resolution limit include the Exposure Latitude (EL), the Dense Isolated Bias (DIB), and the Mask Error Enhancement Factor (MEEF). The Exposure Latitude describes the percentage dose range where the printed pattern's critical dimension (CD) is acceptable, typically 10%. It is used along with the DOF to determine the process window, i.e. the regions of focus and exposure that keep the final resist profile within prescribed specifications. As for the DIB, it is a measure of the size difference between similar features, depending on the pattern density. Finally, the MEEF describes how reticle CD errors are transmitted into wafer CD errors.
As the semiconductor industry moves into the deep submicron regime, the resolution limit of currently available lithographic techniques is being reached due to a decrease in the depth of focus, difficulty in the design of lenses and complexities in the lens fabrication technology. In order to address this issue, there have been continued endeavors to develop resolution enhancement techniques.
Historically, the resolution limit of a lithographic projection tool was optimized by the control of the relative size of the illumination system numerical aperture (NA). Control of this NA with respect to the projection system's objective lens NA allows for modification of spatial coherence at the mask plane, commonly referred to as partial coherence σ. This is accomplished through specification of the condenser lens pupil in a Köhler illumination system. Essentially, this allows for manipulation of the optical processing of diffraction information. Optimization of the partial coherence of a projection imaging system is conventionally accomplished using full circular illuminator apertures. By controlling the distribution of diffraction information in the objective lens with the illuminator pupil size, maximum image modulation can be obtained. Illumination systems can be further refined by considering variations to full circular illumination apertures. A system where illumination is obliquely incident on the mask at an angle so that the zero-th and first diffraction orders are distributed on alternative sides of the optical axis may allow for improvements. Such an approach is generally referred to as off-axis illumination.
Off-axis illumination improves resolution by illuminating the mask with radiation that is at an angle to the optical axis of the lens. The incidence of the radiation on the mask, which acts as a diffraction grating, improves the contrast of the image by transmitting more of the diffracted orders through the lens. Off-axis illumination techniques used with conventional masks produce resolution enhancement effects similar to resolution enhancement effects obtained with phase shifting masks.
Various other enhancement techniques that have been developed to increase the resolution and the DOF include optical proximity correction (OPC) of optical proximity errors (OPE), phase shifting masks (PSM), and sub-resolution assist features (SRAF). Each technique may be used alone, or in combination with other techniques to enhance the resolution of the lithographic projection tool.
One approach to generate off-axis illumination is to incorporate a metal aperture plate filter into the fly eye lens assembly of the projection system illuminator providing oblique illumination. A pattern on such a metal plate would have four symmetrically arranged openings (zones) with sizing and spacing set to allow diffraction order overlap for specific geometry sizing and duty ratio on the photomask. Such an approach results in a significant loss in intensity available to the mask, lowering throughput and making the approach less than desirable. Additionally, the four circular openings need to be designed specifically for certain mask geometry and pitch and do not improve the performance of other geometry sizes and spacings. The previous work in this area describes such a method using either two or four openings in the aperture plate. See, for example, EP 0 500 393, U.S. Pat. Nos. 5,305,054, 5,673,103, 5,638,211, EP 0 496 891 and EP 0 486 316.
Another approach to off-axis illumination using the four-zone configuration, which is disclosed in U.S. Pat. No. 6,452,662 is to divide the illumination field of the projection system into beams that can be shaped to distribute off-axis illumination to the photomask. By incorporating the ability to shape off-axis illumination, throughput and flexibility of the exposure source is maintained. Additionally, this approach allows for illumination that combines off-axis and on-axis (conventional) characteristics. By doing so, the improvement to dense features that are targeted with off-axis illumination is less significant than straight off-axis illumination. The performance of less dense features, however, is more optimal because of the more preferred on-axis illumination for these features. The result is a reduction in the optical proximity effect between dense and isolated features. Optimization is less dependent on feature geometry and more universal illumination conditions can be selected.
Referring to FIGS. 2–5, currently available illumination intensity distributions or arrangements include small, or low, sigma (FIG. 2), annular (FIG. 3), quadrupole (FIG. 4), and quasar (FIG. 5), with the illuminated areas (hereinafter referred to as the aperture(s)) shown in cross section. The annular, quadrupole and quasar illumination techniques of FIGS. 3–5 are examples of off-axis illumination schemes.
Small sigma illumination is incident on the mask with approximately zero illumination angle (i.e., almost perpendicular to the mask) and produces good results with phase shifting masks to improve resolution and increase the depth of focus. Annular illumination is incident on the mask at angles that are circularly symmetrical and improves resolution and increases depth of focus while being less pattern dependent than other illumination schemes. Quadrupole and quasar illumination are incident on the mask with four main angles and provide improved resolution and increased depth of focus while being strongly pattern dependent.
Referring to FIGS. 6 and 7, two illumination systems are schematically illustrated. The systems illustrated in FIGS. 6 and 7 include light collecting/collimating optics 10; an axicon/zoom module 12; and light integrating and projecting optics 14. The illumination systems define an optical axis 16, a pupil plane 18, and a mask plane 20. The axicon/zoom module 12 comprises a pair of axicons 22, one concave and one convex, whose separation can be varied. The module 12 also comprises a zoom lens 24.
For the case of conical axicons, some examples of the illumination intensity distributions achievable at the pupil plane 18 are shown in FIG. 8. The spot size can be varied between states A and B by changing the zoom lens position. Similarly, the annularity can be changed between states A and C by varying the axicon opening (separation between the axicons).
To improve the illumination homogeneity, an optical integrator 26 is used. In FIG. 6 the optical integrator takes the form of a light pipe 26, such as a glass, calcium fluoride or quartz rod. A coupler 28 couples the illumination at the pupil plane 18 into the rod 26, and rod exit imaging optics 30 are also provided. In FIG. 7 a fly's eye element 32 acts as the integrator. The fly's eye element 32 is a composite lens comprising an array or honeycomb of small lenses. Further, objective lenses 34 and 36 complete the projection optics.
It results from the foregoing discussion that numerous options are available to the process engineer for increasing the resolution of the lithographic projection apparatus and for maximizing the process window. Yet, given the cost and time to perform experiments in a semiconductor fabrication facility (fab), the approach that would consist of testing these options across the board to develop a robust process is simply unrealistic. As the product cycle times in the semiconductor field are relatively short, severe limitations are put on the amount of time that can be spent on research and development for each new product line or manufacturing process.
In response to this concern, there has been a growing interest in photolithographic simulations within the industry. Given the fact that a simulation experiment for optimizing the optical proximity correction features for a new mask test may only take one day, instead of months with experiments on wafers, simulations are viewed today as a useful tool to aid in the development and optimization of lithographic apparatuses.
It follows that simulations can be extremely helpful as a development tool, by quickly evaluating options, optimizing processes, and saving time and money by reducing the number of required experiments. Simulations can also aid in the research context to understand many physical phenomena that occur when pushing the limits of resolution to achieve feature size in the order of or below the wavelength of the lithographic apparatus. Finally, simulations can be a tremendous tool to extend the optical lithography to the sub-150 nm regime and to prolong the lifetimes of million dollar steppers and billion dollar fabs.